1. Field of the Invention
The present invention relates generally to a method for manufacturing aluminium alloy parts with precipitation hardening obtained by friction stir welding or FSW. Aluminium alloys with precipitation hardening are denoted in the 2xxx (Al—Cu), 4xxx (Al—Si), 6xxx (Al—Si—Mg), 7xxx (Al—Zn—Mg or Al—Zn—Mg—Cu) or 8xxx (Al—Li—Cu) series according to the Aluminum Association nomenclature. These alloys are hardened by heat treatment including solution heat treatment, quenching and possibly annealing.
2. Description of Related Art
Friction stir welding was initiated in the early 1990s by TWI (The Welding Institute) in the United Kingdom, and has been used in assembling aluminium alloys. Its principle is to obtain a weld without melting the metal, by applying strong shear to the metal using a rotating tool that stirs the two materials to be assembled. First, the yield stress is reduced by heating the metal by applying friction using a shoulder portion of the rotating tool to the metal surface, and the tool is then moved to make the weld by gradually moving it in a forward direction. The shoulder portion of the tool also contains the metal and maintains the pressure to avoid metal ejection outside the welded zone.
The process avoids hot cracking problems, which in particular means that alloys that were previously considered as not being weldable by fusion, can now be welded. For example, 2000 series magnesium alloys or 7000 series copper alloys were previously considered not fusion weldable, but can be subjected to friction stir welding. These alloys are widely used in aeronautical construction, and hence, being able to subject them to friction stir welding is advantageous.
The metallurgical structure inside and around the friction stir welded zone gives a very characteristic facet, which is significantly different from the facet obtained with fusion weld. Apart from zones remote from the weld that are completely unaffected, three distinct zones can be distinguished, as shown in FIG. 1:                1—the zone affected by the most severe plastic deformation is called the nugget. It has a very fine recrystallised microstructure which is relatively equiaxial, with significant decorations at the grain boundaries. During welding, the temperature can reach 560° C. in this zone. It also has an onion skin type annular structure. The width of the nugget is usually slightly more than the tool diameter.        2—the second zone on each side of the nugget is the thermo-mechanically affected zone, which deformed to a lesser extent and which, depending on the alloy, may show signs of recrystallisation.        3—the third zone above the nugget is called the “plastically deformed zone” and is formed by the rotation effect of the tool shoulder.        
Different assembly configurations are possible, but the most frequently used is butt welding.
Friction stir welding leads to very small grains, typically of the order of a few micrometers. These small grains in the as-welded condition contain a high amount of energy stored in the grain boundaries of the welded zone and of the heat-affected zone. This microstructure is therefore unstable.
The weak point of any welded part is the heat-affected zone, regardless of the process used. One known method of eliminating this weak point is to apply solution heat treatment to the welded boundary so as to permit a high mechanical strength at all points. During solution heat treatment, the energy stored in the grain boundaries is released. Hence, the average grain size in the nugget and in the plastically deformed zone is considerably increased, and can be as high as several mm. This uncontrolled grain growth is due to a so-called “secondary recrystallization.” An article by Kh. A. A. Hassan et al “Stability of nugget zone grain structures in high-strength Al-alloy friction stir welds during solution treatment” published in Acta Materialia vol. 51, 2003, pp. 1923-1936 clearly defines this abnormal growth in the grain size, and indicates that solutions for dealing with the same include (i) increasing the density of the dispersoids (which will slow down or block this growth mechanism), or (ii) controlling the heat quantity generated during welding (which will lead to less energy stored in the grain boundaries, and thus to a coarser grain structure).
A coarse grain structure is typically not very favorable for good mechanical behavior, particularly with respect to ductility, fracture toughness and fatigue strength, which makes subsequent shaping difficult and causes inter-granular failures.